• journal_title：The Canadian Mineralogist
• Contributor：Frédéric Hatert ; Luisa Ottolini ; Johan Wouters ; François Fontan
• Publisher：Mineralogical Association of Canada
• Date：2012-08-01
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.50.4.843
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：50
• issue：4
• firstpage：843
• section：Mineralogy, Crystal Structure and Crystal Chemistry

A sample from the Koktokay No. 3 granitic pegmatite, Altai Mountains, northwestern China, which shows a transition from lithiophilite [LiMn²⁺PO₄] to sicklerite [Li_1–xMn²⁺PO₄], was investigated by single-crystal X-ray diffraction, electron-microprobe analysis, and secondary-ion mass spectrometry (SIMS) techniques. Under the polarizing microscope, the sample shows colorless lithiophilite and deep-orange sicklerite, as well as several intermediate phases. The chemical compositions change from Li_0.96(Mn²⁺_0.81Fe²⁺_0.09Fe³⁺_0.08)PO₄ to Li_0.69(Mn²⁺_0.62Mn³⁺_0.19Fe³⁺_0.16)PO₄, and show a progressive decrease of the lithium content from lithiophilite to sicklerite. Five crystals were extracted from the thin section, and their crystal structures were refined in space group Pbnm, with unit-cell parameters from a 4.736(1), b 10.432(2), c 6.088(1) Å (lithiophilite), to a 4.765(1), b 10.338(1), c 6.060(1) Å (sicklerite). The olivine-type structures of these phosphates are identical to that of triphylite, and are characterized by two chains of edge-sharing octahedra parallel to the c axis. The first chain consists of M1 octahedra containing Li atoms and vacancies, and the second chain consists of M2 octahedra occupied by Fe and Mn. The significant Mn³⁺ content in sicklerite necessitates a more careful interpretation of the electron-microprobe data, and demonstrates the different mechanisms of oxidation affecting the lithiophilite–sicklerite and triphylite–ferrisicklerite series, respectively.